In the production of seamless pipes by a Mannesmann-mandrel mill method, first, billets of a raw material are heated in a rotary furnace type heating furnace and then supplied to a rolling line in succession. Specifically, the billet is pierced and rolled using the piercing plug and rolling rolls of a piercing-rolling mill to produce a hollow shell. Next, a mandrel bar is inserted into the hollow shell and the hollow shell is subjected to draw rolling with an outer surface of the hollow shell restricted with grooved rolling rolls of a mandrel mill comprising a plurality of stands, and thereby a thickness of the hollow shell is reduced to a predetermined thickness. Thereafter, the mandrel bar is drawn off and the material pipe with a reduced thickness is sized and rolled by a sizing-rolling mill to reduce an outer diameter to a predetermined outer diameter to obtain a product.
FIG. 1 (FIG. 1A and FIG. 1B) are views showing a schematic constitution of a piercing-rolling mill in which FIG. 1A is a side view thereof and FIG. 1B is a plan view thereof. In addition, the piercing plug is not shown in FIG. 1B.
As shown in FIG. 1, a piercing-rolling mill 10 includes a pair of rolling rolls 1a, 1b, which are inclined to each other, and a cannonball-like piercing plug 3 supported by a mandrel 2 at its rear end. While the pair of rolling rolls 1a, 1b are set in such a way that their axial directions are parallel to each other or cross each other at a predetermined crossed axes angle as viewed from the side, the pair of rolling rolls 1a, 1b are placed in such a way that their axial directions are inclined at inclination angles FA in the directions opposite to each other as viewed from above, and the pair of rolling rolls 1a, 1b are arranged so as to rotate in the same directions. The piercing plug 3 is placed between the pair of rolling rolls 1a, 1b. 
In order to subject a solid billet B to piercing and rolling by using the piercing-rolling mill 10 having a constitution described above, first the billet B is fed to between a pair of rolling rolls 1a, 1b. After the billet B bites the pair of rolling rolls 1a, 1b, a force to rotate and a force to move forward in an axial direction are simultaneously exerted on the billet B by a frictional force of the rolling rolls 1a, 1b. Further, compressive stress and tensile stress is alternately exerted in succession on a central portion of the billet B by the rolling rolls 1a, 1b (This is called “rotary forging effects”.) and the central portion of the billet B becomes a state in which a bore is easily opened before the billet B reaches a tip of the piercing plug 3. When the billet B impinges on the piercing plug 3, a bore is opened at the central portion of the billet B, and then the billet B is subjected to thickness processing in every half turn between the rolling rolls 1a, 1b and the piercing plug 3 to obtain a pipe (hollow shell) S.
In piercing and rolling using the piercing-rolling mill 10 described above, the largest problem relating to the dimensional accuracy of the pipes S produced is the occurrence of the eccentric uneven thickness (primary uneven thickness).
FIG. 2 is a sectional view of a pipe or tube for illustrating an eccentric uneven thickness of the pipe or tube.
As shown in FIG. 2, the eccentric uneven thickness is an uneven thickness (thickness variation) in a circumferential direction of the pipe S, which is produced due to eccentricity (deviation) between a center C1 of the outer surface and a center C2 of the inner surface of the pipe S, and an uneven thickness in which the thickness of the pipe S varies in a cycle of 360 degrees in a circumferential direction.
In order to correct the production conditions of the manufacturing facilities of a seamless pipe such as a piercing-rolling mill quickly so that the occurrence of the eccentric uneven thickness is quickly suppressed, it is effective to measure a thickness distribution in the circumferential direction of a pipe practically on a rolling line such as an exit side of the piercing-rolling mill and to reflect the result of this measurement in correcting the production conditions.
As a method of measuring a thickness distribution in the circumferential direction of a pipe on a rolling line, a method in which a γ ray thickness meter is used is publicly known. However, since the γ ray thickness meter is based on a principle that a thickness is determined based on attenuation of γ rays passing through the pipe, this method has a constraint that a thickness of the pipe cannot be measured in a state in which an instrument such as a piercing plug or a mandrel bar is inserted into the pipe in a location such as the exit side of the piercing-rolling mill or the entrance side of the mandrel mill.
Therefore, a method has been proposed, wherein the γ ray thickness meter is placed on the exit side of a mandrel mill or the entrance side or the exit side of a sizing-rolling mill on which an instrument is not inserted into the pipe to measure a thickness from two or more directions in a plane of a pipe cross section, and production conditions are set/corrected based on the results of this, see for example, Japanese Unexamined Patent Publication No. 8-71616).
However, in a measuring method using a γ-ray thickness meter, if there is a deviation between a core of the γ-ray thickness meter and a core of the pipe, the thickness distribution measured, especially the eccentric uneven thickness, has a large error. In addition, the core of the γ-ray thickness meter is an assumed core. For example, in the case of a γ-ray thickness meter of a multi-beam type disclosed in “TETSU-TO-HAGANE” (No. 9, p. 1139-1145 (1970)), the core of the γ-ray thickness meter means a barycenter position of the respective positions (a position where γ-rays applied from two or more directions intersect) at which the thickness of the pipe is measured. The above-mentioned deviation between cores is unavoidable on the rolling line. In practice, it is difficult to measure the eccentric uneven thickness with high precision before an off-line examination is performed after rolling. Therefore, a waiting period occurs before the results from the off-line examination are available and production conditions cannot be corrected quickly during the manufacturing of a seamless pipe.
Further, since the eccentric uneven thickness of a pipe can result from uneven heat or temperature variations in the circumferential direction of the billet and those resulting from whirling of the piercing plug, it is necessary to correct production conditions according to the respective causes in order to suppress the eccentric uneven thickness. That is, when the eccentric uneven thickness is one resulting from uneven heat in the circumferential direction of the billet, it is necessary to correct the conditions of a firing furnace so as to perform uniform heating. On the other hand, when the eccentric uneven thickness is one resulting from whirling of the piercing plug, it is necessary to take countermeasures such as correction of a core of the rolling rolls of the piercing-rolling mill, disposal of an abnormal piercing plug and the like. Therefore, in order to quickly correct production conditions during the manufacturing of a seamless pipe, it is desired not only to measure the eccentric uneven thickness of a pipe on a rolling line but also to provide a means capable of determining the cause of the occurrence of the eccentric uneven thickness.